Estimate of CO2-temperature feedback suggests lower impact

One of the feedbacks inherent in the Earth's climate system is that changes in …

It's fairly easy to calculate the direct impact of adding more greenhouse gasses to the atmosphere, since that involves basic radiative physics. But, as we mentioned previously, the climate isn't a static system, and changing the temperature induces all sorts of changes that can feed back into the climate. One of these is CO2 itself, as shifts in temperatures can induce alterations in its concentration in the atmosphere. A new paper in today's Nature suggests that this feedback is unlikely to be as large as some of the worst-case estimates.

Estimating feedbacks is notoriously tricky business. Many of them take decades to reach a steady state, and our instrumental record is quite short. For longer time scales, researchers have to rely on temperature reconstructions from proxies, which have their own uncertainties, and only extend for about 1,000 years with any accuracy. Ice cores from Greenland and Antarctica allow us to go longer still, but that record is dominated by the glacial cycle, which is unlikely to be directly relevant to our current situation—in glacial cycles, changes are driven by orbital variations, and the feedbacks from the gain or loss of giant ice sheets are enormous.

So, for better or for worse, the new paper focuses on the previous millennium, specifically the period from 1050 to 1800. That period eliminates earlier times, when the proxy temperature reconstructions are still considered a bit unreliable, and stops before industrialization started having a large impact on the atmosphere's composition. Still, the authors note that the human population was quite significant during this period, and could have driven significant changes in the atmosphere via land use. The estimates for this impact, however, are considered very speculative, so they were not examined as part of the primary analysis.

That analysis gathered a number of proxy temperature reconstructions available in the literature, and used CO2 data from ice cores. The authors recognized that the proxy reconstructions involve a variety of assumptions: the period used for calibration, constraints for amplitude or absolute value of temperature changes, etc. So, to deal with that, they simply ran their analysis once for each possible combination of calibration and smoothing, producing almost 230,000 runs in all.

Obviously, those different runs produced different results, but the authors used them to generate a statistical measure of the most likely feedback on CO2 levels, and to estimate the confidence interval for these measures. For the entire period, the median feedback comes in at 7.7 parts-per-million for each degree (C) of temperature change. The likely range is anywhere between 1.7 to 21.4 ppm per degree.

Earlier estimates had run as high as 200ppm/°C, so that's quite good news in that sense. However, negative values are also unlikely based on this analysis, so there's no indication that some temperature-activated carbon sequestration mechanism (like accelerated plant growth) will help stabilize the climate, at least in the near term.

One caveat to the results came when the authors split the analysis period roughly in half, so that the earlier portion (1050-1549) covered a relatively warm period, while the later one included the Little Ice Age. The warm period was characterized by a lower mean (4.3 ppm/°C) and smaller variation, but the value changed suddenly at the onset of the Little Ice Age, which raised the mean to 16.1 ppm/°C and increased the range of values. All of which suggests that the feedback value may be sensitive to the precise conditions.

I could go on for a while listing other potential caveats—the authors are extremely conservative, and their list of potential confounders is quite long. Their conclusions are bit mixed, as well. They start by saying that their results, combined with a few other recent studies, suggest "reduced possibilities for unwelcome surprises within the next century." But they follow that with a long list of factors that could produce unwelcome surprises in the presence of rapidly rising CO2 levels. So, the take home is a bit of a mixed bag.

To provide one of my own, this seems to represent a solid initial effort, and the analysis performed here will produce better results when provided with less uncertain data regarding ice core measurements, temperature proxies, and the like. Better yet, the news appears to be good: under the conditions that prevailed during the last millennium, increasing temperatures add more CO2 to the atmosphere, but not the sorts of massive quantities that would make this a major feedback. The most significant uncertainties result from the fact that those conditions no longer prevail.